The HIV1 protein Vpr is highly conserved hi primary patient isolates and is, in vitro, important for establishing low titer infections in macrophages and enhancing virus replication in dividing cells. How Vpr unctions in these capacities and in vivo remains poorly understood. The importance of Vpr for primate entivirus infections is underscored by the observation that several of these viruses, including HIV2, have duplicated vpr to generate two open reading frames, vpr and vpx. This duplication has permitted divergence and presumably, further optimization of Vpr functions. The duplication also supports the supposition that SLV1 Vpr has at least two functions. Experimental infections in which Vpr/x production has been disabled demonstrate diminished pathogenicity. Our work has focused on discovering the role that HIV1 Vpr plays by dentifying cellular proteins that it engages. We compare these with the proteins targeted by its HIV2 counterparts under the hypothesis that although these proteins have diverged hi sequence, their functions still overlap as should the identity of then* protein partners. Our preliminary studies, employing co- immunoprecipitation and tandem mass spectroscopy, have revealed that HTV1 and 2 Vpr, like the V proteins of some paramyxoviruses, both engage an ubiquitin ligase complex that includes DDB1. We hypothesize that Vpr, like the V proteins, targets anti-viral factors for proteasomal degradation using DDB1 to recrui ubiquitin ligases. SPECIFIC AIM 1 will focus on testing the ability of Vpr to, like the V-proteins, act as an adaptor to the ubiquitin ligase machinery and thereby to promote degradation of Stat signaling proteins, or to function as an intermediate in the previously described Vpr-mediated destruction of uracil-N-glycosylase SPECIFIC AIM 2 will determine how the Vpr interactions with ubiquitin ligase complexes impact HIV infections. SPECIFIC AIM 3 will focus on determining the identity of targets of Vpr-mediated ubiquitilation important for Vpr-mediated cell cycle arrest and examining the normal function of the complexes that Vpr engages to promote ubiquitilation.